On the Origin of Reversible Hydrogen Activation by Phosphine–Boranes

Abstract

Mechanistic insights into the factors responsible for the reversible hydrogen-activation ability exhibited by an aryl phosphine-borane system ((CH(3))(2)P-C(6)F(4)-B(CF(3))(2)) are presented. A detailed evaluation of the energies of various intermediates, generated by the addition of molecular hydrogen, and their interconverting barriers have been carried out using ab initio and DFT methods. Several rearrangement possibilities of the H(2)-phosphino-borane adduct have been investigated so as to unravel the lower energy pathways that convert the initial adduct to a series of other intermediates. The initial adduct formed by the heterolytic addition of a molecular hydrogen across the C-B bond is identified to undergo a series of rearrangement reactions until it terminates at the C-P end of the molecule. Among the possible 1,n-migrations (for which n=1-5), 1,2-proton migrations are found to possess lower energy transition states, whereas 1,2-hydride (in a zwitterionic intermediate) and 1,4-proton-coupled electron transfers exhibited much higher energy transition states. The minimum energy pathway for the transfer of a proton and hydride from the C-B bond to the C-P bond is found to involve a cascade of 1,2-proton transfers followed by a 1,2-hydride migration and finally a 1,4-proton-coupled electron transfer. The higher energy pathways identified for the hydride transfer suggest the possibility of a cascade of reversible proton migrations from a thermodynamically stable intermediate (M(a)). Possible uptake of two hydrogen molecules by the phosphine-borane system is additionally considered in the present study, in which relatively higher barriers than those with one molecule of hydrogen are observed. The computed thermodynamic parameters are found to be in accordance with the experimental observations, in which the uptake and storage of molecular hydrogen are carried out at lower temperatures whereas the liberation demands elevated temperatures.

The Hydride‐Ion Affinity of Borenium Cations and Their Propensity to Activate H2 in Frustrated Lewis Pairs

“…FLPs containing phenyl‐ligated borenium cations : Mechanistically, extensive computational work indicated that the activation of H 2 by B(C 6 F 5 ) 3 /LBs proceeds through a concerted process involving “encounter complexes” 50. However, an alternative mechanism involving intramolecular heterolytic cleavage of H 2 across a BC bond followed by subsequent intermolecular deprotonation cannot currently be precluded 51. Furthermore, Piers et al.…”

Dihydrogen Activation by Antiaromatic Pentaarylboroles

“…[20][21] This has been proposed as the initial step in the addition of H2 to Stephan's seminal phosphinoborane hydrogen activation system, 4 and supported computationally. 22 This mechanism is conceptually related to that developed for the B(C6F5)3 catalyzed hydrosilation of carbonyl [23][24][25] and imine 26 functions and the dehydrosilation of alcohols. 27 Here, the Lewis acidic borane activates the silane towards nucleophilic attack by the substrate by partially abstracting the silane hydrogen via a borane/silane adduct related to II.…”

Chemistry of Intermolecular Frustrated Lewis Pairs in Motion: Emerging Perspectives and Prospects

“…Since the total association energy of the encounter complex is a sum of interactions that individually are weak – each on the scale of k B T at room temperature – it is doubtful that the LB/LA pair could be confined to the gas phase potential energy minimum for a long period of time at room temperature. That, and results of condensed phase MD simulations using the parameterized force field, have prompted the first ab initio level inquiry into dynamics of the t Bu 3 P/B(C 6 F 5 ) 3 pair at room temperature in the gas phase 45…”